1. Field of the Invention
The present invention relates to a scanning probe type microscope apparatus which is capable of simultaneously observing an optical microscopic image of a scanning probe tip and that of a sample surface.
2. Description of the Prior Art
In recent years, many efforts have been made for the development and the improvement of a scanning probe type microscope apparatus in which the sample surface is scanned in close vicinity to it with a probe and its microstructure is observed by means of detecting a tunnel current, an atomic force or a magnetic force acting between the probe and the sample.
FIG. 5 shows a conceptional representation for explaining the principle of a scanning tunnel microscope (STM), a kind of scanning probe type microscopes. As shown in the figure, a bias voltage is given to either of a conductive sample 101 or a scanning probe 102a, and when they are made to be close to each other in the order of 1 to 2 nm, the electron clouds 120 on both sides are overlapped and a tunnel effect in which electrons pass through the gap between the tip of the scanning probe 102a and the sample 101 is generated and thus a tunnel current flows. Therefore, the value of the current which flows between the scanning probe 102a and the sample 101 can be varied by moving the scanning probe 102a in vertical direction by means of a piezoelectric element 102Z in the direction of Z axis. A three dimensional image of the sample surface can be obtained at a resolution of an atomic level by making a raster scan on the sample surface with piezoelectric elements 102X and 102Y in the directions of X axis and Y axis respectively in keeping the current at a constant value with a feedback circuit (in moving the scanning probe 102a in vertical direction to keep the gap between the scanning probe 102a and the sample 101 constant).
In FIG. 5, the scanning probe 102a so-called a tripod type is shown for easy understanding of the explanation, a probe which is generally used is, however, a tube type probe as shown in FIG. 6. In the case of the probe shown in FIG. 6, an electrode 102G is deposited on the inner peripheral surface of a piezoelectric ceramic member of a cylindrical shape, and on the outer peripheral surface of it, X-direction electrodes 102X1 and 102X2, Y-direction electrodes 102Y1 and 102Y2 and a Z-direction electrode 102Z are deposited. The movement of the probe tip (not shown in the drawing) in three dimensional directions is made possible by selectively applying a voltage to each of the electrodes.
FIG. 7 is a perspective view showing an example of a conventional scanning tunnel microscope using a tube type probe. In the figure, a probe 102 with a probe tip is disposed opposing to a sample holder 109 loaded with a sample 101. The sample holder 109 is moved with a stepping motor 110 and is made to be close enough to the scanning probe 102 till a tunnel current is detected, and an optimum tunnel current is decided according to the current to be detected from the scanning probe 102. A three dimensional image of the surface of the sample 101 can be obtained by scanning the surface of the sample 101 in vertocal moving the probe 102 to maintain the current at a preset value.
In a conventional scanning probe type microscope as described above, however, it is impossible to observe the roughness on a surface of a sample at a resolution of an optical microscope level, so that the following problems are considered to be solved. That is, although the abovementioned scanning tunnel microscope has a high resolution on the one hand, the maximum observing area of the microscope is as narrow as about 10 mm square, so that it is difficult to specify an observing spot, and therefore, such a case may occur as that the tip of the scanning probe hits against a sharp roughness portion on the surface resulting the destroy of the tip, or that a measurement is made without confirming if the sample surface is contaminated.
In a case where a pattern formed on the surface of a substrate, such as a semiconductor wafer for example, is to be observed, although an alignment operation is required to set an observing spot onto a specified position, it is almost impossible to make a precise alignment with only a scanning tunnel microscope which has a narrow observation area.
In order to solve such problems, some proposals have been made as follows.
That is, in one of the proposals, an objective lens of an optical microscope and a tube type scanning probe of a scanning tunnel microscope are mounted on a revolver of the optical microscope, and the observation by the optical microscope and the observation by the scanning tunnel microscope are respectively made by turning the revolver.
In U.S. Pat. No. 4,999,495 granted to Miyata et al., it is disclosed that, for the purpose of solving the problem in the case of mounting both the objective lens and the probe onto the same revolver (such as a difficult wiring for the prove), the objective lens and the probe of the scanning tunnel microscope are supported on a single-axis table so as to face either of the objective lens or the probe to a sample to be observed by moving the single-axis table, or the scanning probe is supported by a frame of the optical microscope so as to make a positioning by an optical microscope and then a sample stage is moved by a specified correction quantity for the alignment of the probe and the sample on the stage.
Further, according to teachings disclosed in U.S. Pat. No. 4,914,293 granted to Hayashi et al., the probe of the scanning tunnel microscope is integrally mounted to the object lens of the optical microscope, or the probe of the scanning tunnel microscope is mounted on a transparent member attached on the lower end of the objective lens, thereby combining the scanning tunnel microscope and the optical microscope.
In the construction proposed in the U.S. Pat. No. 4,999,495, however, the probe is disposed in a position apart from the optical axis of the optical microscope, so that there is a problem that the surface of the sample and the tip of the probe cannot be observed simultaneously.
In the cases of conventional proposals, every one of them has a construction in which the scanning probe is supported on a constituent of the optical microscope, such as an arm or the object lens, on the one hand, the conventional optical microscope has a cantilever structure from the arm to the revolver which holds the object lens. Also a heavy arm of the optical microscope and a mechanism for moving the object lens in vertical direction are weak for vibration, so that it has a low frequency vibration mode in which the resonant frequency lies close to the scanning frequency of the probe tip. Therefore, in the combined construction of the optical microscope and the scanning probe type microscope, if the probe of the scanning probe type microscope is simply fixed to the arm or the optical member of the optical microscope as in the case of the conventional construction, the vibration on the optical microscope is transmitted to the scanning probe, thereby making it difficult to obtain a resolution of an atomic level which is to be a naturally expected level for the scanning probe type microscope. In order to obtain the resolution of the atomic level in the scanning probe type microscope, the noise characteristics caused by the vibration shall be flat over the frequency range of 1 kHz to several tens of kHz and its amplitude shall be within the order of 0.1 angstrom, but it has been impossible to realize such circumstances in any apparatus ever proposed.